1. Field of the Invention
The invention relates generally to switching networks including a plurality of inputs and a plurality of outputs, adapted to route an electrical signal from a selected input to one or more outputs. More particularly, the invention pertains to multi-stage routing switchers, employing special distributive interconnections among plural switching matrices, and further including computerized control systems for routing signals through the matrices, to effect the desired electrical signal routing and distribution.
2. Description of Prior Art
Electrical signal switching networks, or signal routers, are used in a variety of electronic communication applications, such as routing a telephone call from its place of origin to a selected destination. Switching networks are also used to distribute video signals, whether in analog or digital form, from a single input to one or more selected outputs. Thus, for example, a television production studio uses a switching network to distribute a single video signal from a studio camera or a video tape recorder, to a plurality of devices, each having a video input for that signal. These devices would include a video transmitter, another video tape recorder, a monitor, or a video processing unit.
One of the major differences between telephone switchers and video switchers is the requirement that video and audio switchers used for television production have the capability to distribute a single input signal to one or a plurality of selected outputs. Also, video and audio switchers should be "non-blocking", in the sense that the selected distribution of the signal can always be effected by the switcher, and not thwarted by preexisting circuit commitments. Telephone switchers, on the other hand, are commercially acceptable if blocking the desired signal routing occurs on an occasional basis. While an infrequent "busy signal" may be tolerated for commercial telephone operations, it is not acceptable for video switching applications where the inability properly to route a commercial or a live satellite feed may prove very costly.
The most basic form of a switching network is a "two by one", or "single bus" switch. Such a switch includes two inputs and a single output. In this most basic configuration, it has two crosspoints, or switch points, equal to the number of input sources. If another output is added to this basic switching network, the required number of crosspoints doubles.
If this switching network is expanded, for example, to a single stage, square matrix array, having an equal number of inputs N and outputs N, the product of inputs N and outputs N (N.sup.2) determines the total number of crosspoints required for the matrix. Since increasing the number of inputs and outputs results in an exponential increase in the number of crosspoint switches, large scale switchers of this design, whether rectangular or square in configuration, require an unacceptably high number of crosspoint switches. Moreover, it can mathematically be demonstrated that where the number of both the inputs and the outputs required by the network exceeds thirty-six, different arrangements of plural matrices will reduce the total number of switches, or crosspoints in the system, compared to a single stage matrix switcher.
The prior art also teaches more complex switching networks, known as multi-stage routers, which use various numbers and arrangements of switching matrices in combination. These multi-stage routers display the benefit of reduced numbers of crosspoint switches, for large scale switching networks. Multi-stage routers also provide better, but not absolute protection, against signal blocking, when called upon to perform distributive signal routing to more than one output.
In a seminal article entitled, "A Study of Non-Blocking Switching Networks", published in the March, 1953 issue of The Bell System Technical Journal, Charles Clos explained the characteristics and advantages of multi-stage switching arrays. Today, Clos multi-stage signal routers are widely known in the industry, and many commercially available routers are based upon Clos' initial design. As described above, when these switchers are called upon to perform the distributive signal routing required by the television industry, they still may block. This signal blocking may in some instances be cured, because control systems are capable of re-assigning, or re-arranging circuits through a different set of switch interconnections, relieving the temporary blocking condition which existed. Other times, re-assignment is ineffective in establishing a new circuit route, and the signal blocking is then absolute.
In U.S. Pat. No. 4,566,007 issued to Richards, a two-stage, re-arrangeable switching network is disclosed. Richards teaches a special "connection arrangement", posited between the input channels and the first stage switching matrices. In FIG. 3 of Richards, it should be noted that each input channel input is interconnected to two different 5.times.1 matrix switches. These 5.times.1 switches may be divided into two groups, a first group including switches 101-105, and a second group including switches 106-110.
Making reference to input IC1, for example, note that it is interconnected both to a first input of switch 101 and to a first input of switch 106. IC2, however, is interconnected to a second input of switch 101, and to a first input of switch 107. It is evident that connections to the first group of switches are made in a sequential, distributive fashion, each successive input channel being fed to the next sequentially available input terminal among the first group of switches.
Connections to the second group of switches (106-110) are quite different, being made in a non-repetitive, distributive manner. Thus, numerically adjacent input channels display a non-repetitive connection characteristic, by not repeating connections to the same switch of the second group. Moreover, there is a predetermined numerical increment, or pattern, between successive connections for the input channels. This is different than the sequential increment connections, characteristic of the first group of switches. It is this difference between the nature of the interconnections for the first and second groups of switches which prevents blocking for any combination of input and output signal routing selection.
In Karp, U.S. Pat. No. 5,469,154, both two and three-stage re-arrangeable switching networks are shown. In the three-stage network shown in FIG. 2, each output of each first-stage switching "crossbar", or matrix, connects to two second-stage matrices. Karp describes his interconnection pattern between the first and second stages as based upon "addition mod 15". The interconnections between the second and third stages are similar to that of a standard Clos network, in that each third-stage matrix receives one input from each second-stage matrix.
In Karp's two-stage network shown in FIG. 6, the inputs to the first-stage of the switching network are connected to two separate groups of first stage switches, called "upper crossbars" and "lower crossbars". In that regard, Karp's input circuitry is similar in principle to that shown in Richards '007, described above. Interconnections between the first and second stages are analogous to that of a standard Clos network, with all eighteen second stage "crossbars" receiving individual outputs from both upper and lower "crossbars".
Both the Richards and the Karp switching networks use a number of rectangular matrices of various sizes, having unequal numbers of inputs and outputs. The efficient manufacture of a large switching network logically calls for the use of as many common components as possible. It is an object of the present invention, therefore, to provide a switching network incorporating the same size matrix, or sub-switcher component, for all elements of all stages of the router.
In prior art switching networks, the matrices are interconnected in such a way that the failure of one or more matrices may adversely affect overall operation of the network. Accordingly, it is another object of the present invention to provide a network architecture having redundant banks, or gangs, each including primary, secondary, and tertiary switching matrices, interconnected in such a way as to minimize or avoid adverse effects of a failure of one or more of the matrices, or failure of the power supply or control system for one of the banks of matrices.
A need also exists for a large scale, non-blocking electrical signal router, which uses a minimum number of crosspoints or switches, and requires fewer switching reassignments by its signal routing control system than prior art devices. It is therefore a further object of the present invention to satisfy such a need, by providing a signal router having a dual-bank array of switch matrices, each bank having primary, secondary, and tertiary stages, and in which various combinations of sequential distributive and non-repeating distributive interconnections are made, to the input and output circuits of both banks.